Yes, there is Android app support. Firefox should work without major problems (I use it rarely.) But it's kind of backwards, first offering a Gnu/Linux phone, but then a fundmental application like web browser should be run under Android.
It runs a container within which runs AOSP. Cal/CardDAV sync is provided with (official) plugins for the stock Calendar/Contacts apps, mobile hotspot is built-in in Settings, and call record is built-in in Phone app.
Seriously, it already supports everything I need? I'll start taking a closer look. Thank you.
Is it actually run in an Android VM?
I use an OS as a step to run applications. Why would someone use Sailfish if it can not run applications, or do they expect mobile development firms to start supporting a distant third OS? Serious question, there is a real chicken and egg problem here.
I think that I have simple needs from a mobile OS. CalDAV and CardDAV sync, provide a mobile hotspot, record calls. On Android those functions are provided by applications - so I consider myself a user of these applications, not a user of Android. I'm probably the ideal customer for a Sailfish OS, so long as it provides these features or can run apps that do.